Schottky gated transistor with interfacial layer

ABSTRACT

A Schottky gated transistor having reduced gate leakage current is disclosed. The Schottky gated transistor includes a substrate and a plurality of epitaxial layers disposed on the substrate. Further included is a gate contact having an interfacial layer disposed on a surface of the plurality of epitaxial layers and having a thickness that is between about 5 Angstroms (Å) and 40 Å. The interfacial layer can be made up of non-native materials in contrast to a native insulator such as silicon dioxide (SiO 2 ) that is used as an insulating gate layer with silicon-based power transistors. The Schottky gated transistor further includes at least one metal layer disposed over the interfacial layer. A source contact and a drain contact are disposed on the surface of the plurality of epitaxial layers, wherein the source contact and the drain contact are spaced apart from the gate contact and each other.

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application No. 62/008,900, filed Jun. 6, 2014, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure pertains to reducing gate leakage current in high-power transistors.

BACKGROUND

Schottky gate gallium nitride (GaN) transistors used in power switching applications suffer from undesirably high gate leakage current, which results in lower operational efficiency due to excessive power dissipation. Reverse gate leakage for a Schottky contact on a GaN device is typically 10 μA to 1 mA per millimeter (mm) of gate width depending on the construction of the device. For a Schottky gated GaN power transistor with 100 mm gate periphery, the gate-to-drain leakage would be on the order of 1 mA to 100 mA. In contrast, silicon-based power transistors having a dielectric deposited under the gate to provide an “insulated gate” have a much lower leakage current that ranges from about 1 nA to around 100 nA. Moreover, silicon has a natural advantage of an extremely high-quality native oxide, which is silicon dioxide (SiO₂). As such, silicon metal oxide field effect transistors (Si MOSFETs) and insulated gate bipolar transistors (IGBTs) are relatively easily fabricated. Consequently, Si MOSFET devices and IGBT devices dominate the power transistor market. For many years, alternative semiconductors fabricated using wide bandgap semiconductors such as silicon carbide (SiC) and GaN have been evaluated in an attempt to achieve better performance than silicon devices. In most cases, a primary obstacle has been a lack of a high-quality insulator material.

Gallium nitride (GaN) metal oxide semiconductor—high electron mobility transistors (MOSHEMTs) using dielectrics such as SiO₂, silicon nitride (SiN), hafnium dioxide (HfO₂), aluminum oxide (Al₂O₃), and aluminum nitride (AlN) have been proposed, but the performance and reliability of these dielectrics have been limited by the poor interface properties of dielectrics on GaN. One problem is a large hysteresis in current-voltage (I_(D)-V_(G)) transfer characteristics after an application of drain voltage. Another problem is time-dependent gate oxide breakdown (TDDB) caused by relatively large reverse bias voltages applied during operation. Still other problems include threshold voltage instability and mobility degradation due to columbic scattering.

Compounding these problems is a need for an undesirably thick dielectric to support high electric fields that occur in high-voltage GaN transistors that operate under drain voltages that range between 600V and 1200V. For example, a typical high-voltage GaN MOSHEMT typically has an insulating gate layer with a thickness that ranges between 100 Angstroms (Å) and 500 Å. Further still, for a given density of trapped interface charge (Qit) a thick dielectric with a relatively low areal capacitance (Cox) results in a significant threshold voltage (Vth) shift (Δ) that is governed by the following mathematical relationship.

ΔVth=Qit/Cox  EQ. 1

As illustrated by EQ. 1, a larger ΔVth occurs for a given Qit as Cox decreases as the insulating gate layer gets thicker. As a result, performance of a GaN MOSHEMT is degraded with increased gate layer thickness. At some point, the Vth is shifted too much to be practical. Thus, there exists a need for a low-leakage gate for GaN transistors that does not suffer from the disadvantages of insulated gate structures.

SUMMARY

A Schottky gated transistor having reduced gate leakage current is disclosed. The Schottky gated transistor includes a substrate and a plurality of epitaxial layers disposed on the substrate. Further included is a gate contact having an interfacial layer disposed on a surface of the plurality of epitaxial layers and having a thickness that is between about 5 Angstroms (Å) and 40 Å. In at least some embodiments, the interfacial layer is made up of non-native materials in contrast to a native insulator such as silicon dioxide (SiO₂) that is used as an insulating gate layer with silicon-based power transistors.

The Schottky gated transistor further includes at least one metal layer disposed over the interfacial layer. A source contact and a drain contact are disposed on the surface of the plurality of epitaxial layers, wherein the source contact and the drain contact are spaced apart from the gate contact and each other. A benefit of embodiments of the present disclosure is that the interfacial layer provides forward conduction characteristics that are substantially the same as traditional Schottky gated transistors while gate leakage current under reverse gate voltage conditions is reduced by several orders of magnitude in comparison to traditional Schottky transistors.

Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description, serve to explain the principles of the disclosure.

FIG. 1 is a cross-sectional diagram depicting the structure of a Schottky gated transistor with an interfacial layer in accordance with the present disclosure.

FIG. 2 is a graph of current-voltage (I-V) curves comparing the gate current of Schottky gated transistor with the interfacial layer to a Schottky gated transistor without an interfacial layer.

FIG. 3 is a graph of gate I-V curves for a related art silicon dioxide (SiO₂) metal oxide semiconductor heterostructure field effect transistor (MOSHFET) having a relatively thick gate insulator compared to a Schottky gate HFET (HFET is another term for HEMT).

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

This disclosure provides a Schottky gated transistor with an interfacial layer comprising a gate contact that substantially reduces gate leakage current in comparison to traditional Schottky gated transistors. Moreover, the disclosed interfacial layer is ultra-thin having an atomic scale thickness that allows carrier tunneling, which prevents persistent charge trapping.

FIG. 1 is a cross-sectional diagram depicting the structure of a Schottky gated transistor 10 that is structured in accordance with the present disclosure. The Schottky gated transistor 10 includes a substrate 12 over which epitaxial layers 14 are disposed. At least one of the epitaxial layers is a gallium nitride (GaN) based layer. In at least one embodiment, the Schottky gated transistor 10 is a power transistor that is adapted for power switching applications. In one embodiment, the Schottky gated transistor 10 has a drain-to-source breakdown voltage that ranges from greater than 200V to around 600V. In another embodiment, the Schottky gated transistor 10 has a drain-to-source breakdown voltage that ranges from greater than 600V to around 1200V.

A gate contact 16 includes an interfacial layer 18 that is disposed on a surface 20 of the epitaxial layers 14. The interfacial layer 18 is made up of a substantially thin layer of relatively high-resistivity material. In some embodiments, the interfacial layer 18 is deposited using atomic layer deposition of aluminum oxide (Al₂O₃), aluminum nitride (AlN), or aluminum oxynitrides (AlO_(x)N_(y)). In some embodiments, the interfacial layer 18 is deposited using atomic layer deposition or pulsed chemical vapor deposition of silicon oxide (SiO₂), silicon nitrides (SiN_(x)), or silicon oxynitrides (SiO_(x)N_(y)). More extensive lists of exemplary compounds that are usable as the interfacial layer 18 are given further below.

Independent of the material used to make up the interfacial layer 18, the thickness of the interfacial layer is less than 40 Angstroms (Å) but greater than about 5 Å. In at least some embodiments, the thickness of the interfacial layer 18 is between about 5 Å and 15 Å. In other embodiments, the thickness of the interfacial layer 18 is between 15 Å and 25 Å. Yet in other embodiments, the thickness of the interfacial layer 18 is between 25 Å and 40 Å.

The gate contact 16 further includes Schottky metal layers, which in this exemplary embodiment include a first metal layer 22 disposed on the interfacial layer 18, a second metal layer 24 disposed on the first metal layer 22, and an Nth layer 26 disposed over the second metal layer 22. The Schottky metal layers are made up of metals that include, but are not limited to, nickel (Ni), platinum (Pt), palladium (Pd), platinum silicide (PtSi), gold (Au), titanium (Ti), iridium (Ir), and iridium oxide (IrO₂).

The Schottky gated transistor 10 also includes a source contact 28 and a drain contact 30, which are both disposed on the surface 20. The gate contact 16, source contact 28, and the drain contact 30 are spaced apart along the surface 20.

A typical GaN-type Schottky gated transistor without the interfacial layer 18 will have a leakage current under reverse gate voltage conditions that ranges from around 10 μA to around 1 mA per mm of gate periphery. The interfacial layer 18 is adapted to substantially reduce leakage current in the GaN-type Schottky gated transistor 10 under reverse gate voltage conditions. The interfacial layer 18 substantially reduces leakage current while maintaining practically the same forward conduction current-voltage (I-V) characteristics of the typical GaN-type Schottky gated transistor that does not include the interfacial layer 18. The reduction of leakage current under reverse gate voltage conditions is on the order of 1 to 3 orders of magnitude. Exemplary embodiments of interfacial layer 18 reduce leakage current to within a range of on the order of 1 nA to about 100 nA per mm of gate periphery under reverse gate voltage conditions that range between −1 V to around −20V.

Another benefit provided by the interfacial layer 18 is that a brief period of forward conduction through a gate diode formed between the gate contact and the surface 20 when the Schottky gated transistor 10 is switched on prevents persistent charge trapping and hysteresis. In addition, the relative thinness of the interfacial layer 18 makes it difficult for charges to become trapped. Thus, switching performance of the Schottky gated transistor 10 is enhanced by incorporation of the interfacial layer 18.

A first exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, aluminum oxide (Al₂O₃), bismuth trioxide (Bi₂O₃), calcium hafnium oxide (CaHfO), calcium oxide (CaO), cerium(III) oxide (Ce₂O₃), and copper oxides (CuO_(x)). A second exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, dysprosium(III) oxide (Dy₂O₃), erbium oxide (ErO₃), europium(III) oxide (Eu₂O₃), iron(III) oxide (Fe₂O₃), gallium(III) oxide (Ga₂O₃), and gadolinium(III) oxide (Gd₂O₃). A third exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, hafnium oxide (HfO₂) holmium(III) oxide (Ho₂O₃), indium(III) oxide (In₂O₃), lanthanum oxide (La₂O₃), lutetium(III) oxide (Lu₂O₃), and magnesium oxide (MgO).

A fourth exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, nickel monoxide (NiO), niobium pentoxide (Nb₂O₅), lead monoxide (PbO), praseodymium(III) oxide (Pr₂O₃), platinum oxides (PtO_(x)), rhodium(III) oxide (Rh₂O₃), and ruthenium oxide (RuO_(x)). A fifth exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, scandium(III) oxide (Sc₂O₃), silicon dioxide (SiO₂), samarium(III) oxide (Sm₂O₃), tin dioxide (SnO₂), strontium oxide (SrO), and tantalum(V) oxide (Ta₂O₅). A sixth exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, titanium dioxide (TiO₂), thulium(III) oxide (Tm₂O₃), vanadium oxides (V₂O_(x)), tungsten oxide (WO_(x)), yttrium oxide (Y₂O₃), zinc oxide (ZnO), and zirconium dioxide (ZrO₂).

A seventh exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, aluminum nitride (AlN), cobalt nitride (CoN), nitrified copper (CuN_(x)), hafnium nitride (Hf₃N₄), molybdenum nitride (MoN), nickel nitrides (NiN_(x)), silicon nitrides (SiN_(x)), tantalum nitride (TaN), titanium nitride (TiN), and zirconium nitride (Zr₃N₄). An eighth exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, lanthanum fluoride (LaF₃) and zinc fluoride (ZnF). A ninth exemplary list of compounds for making up the interfacial layer 18 includes, but is not limited to, lead sulfide (PbS), cadmium sulfide (CdS), cupric sulfide (CuS), manganese sulfide (MnS), tin(II) sulfide (SnS), and zinc sulfide (ZnS). An tenth exemplary list of compounds for making up the interfacial layer includes, but is not limited to, oxynitrides of the compounds in the first exemplary list through the ninth exemplary list above.

FIG. 2 is a graph of current-voltage (I-V) curves comparing gate current of the Schottky gated transistor 10 (FIG. 1) with the interfacial layer 18 (FIG. 1) with gate current for a Schottky gated transistor (not shown) without an interfacial layer. Notice that with positive gate voltage applied, gate current for the Schottky gated transistor 10 with the interfacial layer 18 as presently disclosed has practically the same I-V curve forward conduction as a traditional Schottky gated transistor without the interfacial layer 18. In contrast, with a reverse gate voltage applied the Schottky gated transistor 10 with the interfacial layer 18 shows a 1 to 3 order of magnitude reduction in gate leakage current in comparison to the traditional Schottky gated transistor without the interfacial layer 18.

FIG. 3 is a graph of I-V curves for a related art SiO₂ metal oxide semiconductor heterostructure field effect transistor (MOSHFET) having a relatively thick gate insulator compared with a Schottky gate heterostructure field effect transistor (HFET). Note that for the purpose of this disclosure, HFET is another term for high electron mobility transistor (HEMT). Both the related art MOSHFET and the related art HFET each have a gate length L_(G)=50 μM. As illustrated by the region in the dashed circle, the related art MOSHFET shows a substantially altered I-V curve forward conduction characteristic compared with the I-V curve forward conduction characteristic for the Schottky gate HFET. This is a result of either using a specific material for an insulating layer having a poor interface with the surface 20 and or increased material thickness for the insulating layer needed to achieve a satisfactory low gate leakage current. As the material thickness for the insulating layer increases, the forward conductance gets lower, as indicated by the downwardly sloping arrow within the dashed circle, indicating an increasingly MOSHFET-like transistor. A take away from FIG. 3 is that the interface layer 18 (FIG. 1) provides superior reduced gate leakage compared with the Schottky gate HFET of FIG. 3 under reverse gate voltage conditions, while maintaining an unaltered I-V forward conduction characteristic and avoiding the disadvantages of the MOSHFET-like devices (charge trapping and TDDB in the thicker insulating layer that is serving as the gate dielectric). In particular, the Schottky gated transistor forward conduction characteristics of a gate diode formed under the gate contact are substantially unchanged in comparison with forward conduction characteristics of a gate diode formed under a gate contact that does not include the interfacial layer.

Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

What is claimed is:
 1. A Schottky gated transistor comprising: a substrate; a plurality of epitaxial layers disposed on the substrate; a gate contact comprising: an interfacial layer disposed on a surface of the plurality of epitaxial layers and having a thickness that is between about 5 Angstroms (Å) and 40 Å; and at least one metal layer disposed over the interfacial layer; a source contact disposed on the surface of the plurality of epitaxial layers; and a drain contact disposed on the surface of the plurality of epitaxial layers, wherein the source contact and the drain contact are spaced apart from the gate contact and each other.
 2. The Schottky gated transistor of claim 1 wherein at least one of the epitaxial layers is gallium nitride (GaN) based.
 3. The Schottky gated transistor of claim 1 wherein a drain-to-source breakdown voltage ranges between about 200V and 600V.
 4. The Schottky gated transistor of claim 1 wherein a drain-to-source breakdown voltage ranges between about 600V and 1200V.
 5. The Schottky gated transistor of claim 1 wherein the thickness of the interfacial layer is between about 5 Å and 15 Å.
 6. The Schottky gated transistor of claim 1 wherein the thickness of the interfacial layer is between about 15 Å and 25 Å.
 7. The Schottky gated transistor of claim 1 wherein the thickness of the interfacial layer is between about 25 Å and about 40 Å.
 8. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of bismuth trioxide (Bi₂O₃), calcium hafnium oxide (CaHfO), calcium oxide (CaO), cerium(III) oxide (Ce₂O₃), and copper oxides (CuO_(x)).
 9. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of dysprosium(III) oxide (Dy₂O₃), erbium oxide (ErO₃), europium(III) oxide (Eu₂O₃), iron(III) oxide (Fe₂O₃), gallium(III) oxide (Ga₂O₃), and gadolinium(III) oxide (Gd₂O₃).
 10. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of dysprosium(III) oxide (Dy₂O₃), erbium oxide (ErO₃), europium(III) oxide (Eu₂O₃), iron(III) oxide (Fe₂O₃), gallium(III) oxide (Ga₂O₃), and gadolinium(III) oxide (Gd₂O₃).
 11. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of holmium(III) oxide (Ho₂O₃), indium(III) oxide (In₂O₃), lanthanum oxide (La₂O₃), lutetium(III) oxide (Lu₂O₃), and magnesium oxide (MgO).
 12. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of nickel monoxide (NiO), niobium pentoxide (Nb₂O₅), lead monoxide (PbO), praseodymium(III) oxide (Pr₂O₃), platinum oxides (PtO_(x)), rhodium(III) oxide (Rh₂O₃), and ruthenium oxide (RuO_(x)).
 13. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of scandium(III) oxide (Sc₂O₃), samarium(III) oxide (Sm₂O₃), tin dioxide (SnO₂), strontium oxide (SrO), and tantalum(V) oxide (Ta₂O₅).
 14. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of titanium dioxide (TiO₂), thulium(III) oxide (Tm₂O₃), vanadium oxides (V₂O_(x)), tungsten oxide (WO_(x)), yttrium oxide (Y₂O₃), zinc oxide (ZnO), and zirconium dioxide (ZrO₂).
 15. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of cobalt nitride (CoN), nitrified copper (CuN_(x)), hafnium nitride (Hf₃N₄), molybdenum nitride (MoN), nickel nitrides (NiN_(x)), silicon nitrides (SiN_(x)), tantalum nitride (TaN), titanium nitride (TiN), and zirconium nitride (Zr₃N₄).
 16. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of lanthanum fluoride (LaF₃) and zinc fluoride (ZnF).
 17. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of lead sulfide (PbS), cadmium sulfide (CdS), cupric sulfide (CuS), manganese sulfide (MnS), tin(II) sulfide (SnS), and zinc sulfide (ZnS).
 18. The Schottky gated transistor of claim 1 wherein a material making up the interfacial layer is selected from the group consisting of aluminum oxide (Al₂O₃), aluminum nitride (AlN), silicon dioxide (SiO₂), and hafnium oxide (HfO₂).
 19. The Schottky gated transistor of claim 1 wherein the at least one metal layer is selected from the group consisting of nickel (Ni), platinum (Pt), palladium (Pd), gold (Au), and titanium (Ti).
 20. The Schottky gated transistor of claim 1 wherein the at least one metal layer is selected from the group consisting of iridium (Ir) and iridium oxide (IrO₂).
 21. The Schottky gated transistor of claim 1 wherein forward conduction characteristics of a gate diode formed under the gate contact are substantially unchanged in comparison with forward conduction characteristics of a gate diode formed under a gate contact that does not include the interfacial layer.
 22. The Schottky gated transistor of claim 1 wherein the interfacial layer reduces gate leakage current to within a range of 1 nA to about 100 nA per millimeter (mm) of gate periphery under reverse gate voltage conditions that range between −1V to around −20V. 